This invention relates to dual-fluid turbine engines, and more particularly, to a flow control system for a dual-fluid gas turbine engine that utilizes a heat-recovery steam generator to generate steam for power augmentation.
A recuperative dual-fluid engine (RDFE) is a gas-turbine engine in which steam generated by a heat-recovery steam generator (HRSG) is used for power augmentation. Either parallel-compound, dual-fluid engines, wherein waste-heat steam is injected into their working fluid, or series-compound, dual-fluid engines wherein power is augmented by an expander operating solely on waste-heat steam are the usual methods to achieve power augmentation with steam.
The present invention relates to the parallel-compound RDFE wherein waste-heat steam is injected into the working fluid of a dual-fluid, gas turbine engine used for ship propulsion, the part-power performance of the gas turbine engine being optimized by means of a highly versatile steam-rate control system.
Over the past few decades, the land-based power industry has exhibited an increasing use of RDFE-type plants. This commercial development has been spurred primarily by the fact that the thermal efficiency of the RDFE is markedly better than that of the simple-cycle engine. Theoretical investigations of the performance of parallel-compound dual-fluid engines at high steam/air mass ratios have confirmed that the RDFE has potential for substantial increases in both power density and specific power. However, the development that markedly enhanced the engine""s suitability for propulsion applications was described in U.S. Pat. No. 4,128,994 issued to Cheng, which is hereby incorporated by reference, that showed that when, at any point on the predetermined power-profile curve of the RDFE, the flow of a second parallel working fluid (steam) is controlled so as to produce the maximum degree of superheating, the thermal efficiency of the RDFE is also maximized.
For many RDFE propulsion applications, the most attractive type of HRSG is the Forced-Recirculation Boiler (FRB). This is partially due to the compactness afforded by the forced-recirculation flow system incorporated into its design. Moreover, this type of RSG is inherently superior to unrecirculated boilers from the standpoint of uniformity of flow distribution and the control of both the dissolved solids in the boiler water and the superheat temperature of the product steam. The FRB generally has an economizer, evaporator, and superheater that generally consist of a plurality of heat-transfer surfaces such as finned tubes. The flow system of FRBs is well known to those skilled in the art.
FIG. 3 illustrates the prior art principles of FRB operation. Feedwater-pump 80 delivers pressurized water from the water supply to economizer 81, which heats the water to a temperature slightly below the saturation condition and delivers it to steam-drum 84. The steam-drum is made up of a steam separator and a quantity of water equal to about two-thirds of the steam drum volume. Where control of the tube-wall temperature of the economizer is not required, the flowrate of the economizer water is only slightly higher than that of the steam leaving the steam drum 84, with the excess compensating for the water lost in continuous blowdown of the steam drum. Blowdown is critical to the control of dissolved solids that collect at the bottom of the steam drum. In cases where tube-wall temperature control is needed, the flow through the economizer 81 is increased to provide a drum-bypass stream for raising the temperature of the feedwater delivered to the FRB. Recirculation pump 85 delivers water from the steam-drum 84 to the inlet of evaporator 82, which vaporizes a portion of the water and delivers the water-steam mixture to the steam separator; the separator is effective for collecting the saturated steam and sending it to superheater 83. By operating the evaporator at a flow rate much higher than that in the superheater 83, the steam quality of the evaporator 82 water-steam mixture remains very low, thus avoiding local hot spots that could lead to eventual burnout of the tube wall. The water flows through the FRB in a multipass counter-current manner with respect to the flow of hot gas sent directly to the FRB from the RDFE expander.
Because part-power performance optimization is an inherent requirement. for propulsion engines, the design of a FRB for propulsion must provide both high steam-turn-down ratio and maximum heat recovery (low gas-exit temperature) over a wide power range of the engine. However, in satisfying these requirements, problems relating to both boiler flow stability and operational life are introduced. It is, therefore, an object of the present invention to provide a mode of boiler operation which will enable, under operational requirements, the delivery of the predetermined steam rates needed for optimizing the RDFE performance at particular points on the predetermined power-profile curve. It is recognized that there is a need for controlling gas-side cold corrosion to extend operational life and for maintaining boiler flow stability for any predetermined off-design steam rate.
In the present invention the flow system of the FRB is modified to maintain the stability and integrity of the boiler by simultaneously providing, under any predetermined off-design, gas-side flow conditions, means for (1) limiting the gas-side cold corrosion of the economizer tubes through tube-wall temperature control, and (2) introducing a controllable sensible component into the heat load of the evaporator, thereby enabling, for any predetermined off-design steam rate, stable evaporator operation at a predetermined design steam quality.
One embodiment of the invention is achieved by adding a recirculation loop to the flow circuit of the economizer. This enables, through diversion of the water flows within the loop, the conditioning of the flows to both the economizer and the steam drum. Simulation of the operation of the flow-system invention along the power-profile curve of the engine has demonstrated the efficacy of the flow-system in enabling, at any particular point on the power-profile curve, the predetermined steam rate needed for optimum performance of the engine.